1. Field of the Invention
The present invention relates to a positive working photoresist composition, and more particularly to a photoresist composition containing a polyphenol and a sensitizer.
2. Description of the Prior Art
Photoresist compositions are facing new challenges in the fabrication of microelectronic components. The need to mass-produce submicron circuits has resulted in a demand for resists with the properties of high resolution, high thermal resistance, high plasma resistance and optimal dissolution characteristics.
When producing a microelectronic component such as a microchip, the amount of information which can be placed on the chip is a direct function of the resolution available with the photoresist. As the resolution of the photoresist increases, higher storage capacities are attainable in the microelectronic component. Accordingly, there is an ongoing effort in the industry to produce photoresists which are capable of higher resolution.
Positive-working photoresist materials evolved from discoveries made by the Kalle Corporation which developed the first positive-acting photoresist based on the use of a novolak matrix resin and a diazoquinone photo-active compound or sensitizer which retards dissolution of the novolak. To produce the Kalle photoresist, the sensitizer and novolak resin are dissolved in an organic solvent and the resultant solution is coated onto a substrate such as a silicon wafer using methods known in the art such as spin coating. The coating solvent is then removed from the resist composition to produce a film on the substrate in which the sensitizer is uniformly distributed throughout the novolak matrix. Upon exposure of the photoresist composition to radiation, the sensitizer is converted into a base-soluble, acidic photoproduct that does not retard dissolution of the novolak in the exposed regions. After exposure, the photoresist is selectively dissolved in an aqueous base to remove the areas of the photoresist which were subject to the image-wise exposure. Images produced from positive-working resists are extremely accurate, require minimal processing techniques and involve few processing steps. Examples of such positive-working photoresist compositions are disclosed in UK Patent No. 784,672; U.S. Pat. No. 3,402,044; U.S. Pat. No. 4,365,019 and U.S. Pat. No. 4,684,599. The resins most often used with positive-resist sensitizers are the phenol formaldehyde novolak and its derivatives and poly(p-vinylphenols).
Positive-working photoresist compositions as described above are used in the semiconductor chip industry. After removing the exposed portions of the resist pattern, the underlying substrate is etched. Dry etching or plasma etching techniques are now commonly used to achieve the submicron resolution that is required to produce microchips having storage capacity which approaches and exceeds one megabyte. Resins having sufficiently high thermal stability and plasma resistance to be used with these high resolution techniques are therefore required.
Two plasma etching or resist removal techniques have been used in the microchip industry. In the single layer resist technique, a single layer of a photoresist is applied to the surface of a silicon chip. The resist is exposed and then developed with an alkaline solution to remove the exposed portions of the resist. The substrate is then plasma etched using a CF.sub.4 plasma. The areas of the substrate which are not protected by the resist are removed. In a second process, known as the multilayer resist process, a photoresist containing silicon is coated over a layer of a second resist material on a substrate. The second resist may be photosensitive or non-photosensitive. The silicon-containing resist is exposed and developed with an alkaline developer solution to uncover areas of the underlying resist. The multilayer resist is then subjected to an oxygen plasma which converts the silicon present in the upper resist to silicon dioxide and removes the uncovered areas of the underlying resist.
Attempts have been made to improve the thermal stability and plasma resistance of novolak resists. Following development resists have been heated to above their cross-linking temperature to produce a thermally stable structure. However, the cross-linking temperature of novolak resists is much higher than the glass transition temperature of the resists, creating a thermally stable resist profile only after severe reflow.
Other well-known hardening techniques consist of cross-linking the novolak resin resist by irradiation with deep UV light or by subjecting the image resist to a noneroding plasma. These techniques harden the resist image only at the surface because of limited penetration of the cross-linking action. Surface hardening works well with small features With larger features, the skin is not sufficiently rigid to hold the bulk of the resist which deforms at temperatures above the glass transition temperature. Hardening by electron beam or ion beam bombardment can crosslink the resist in the bulk but there is a possibility of radiation damage of the semi-conductor devices on the wafer as well as high equipment costs.
Further refinements include progressive heating of the resist image while it undergoes skin hardening, but extremely sophisticated microprocessor control must be implemented and the process must be reoptimized frequently.
It is also known in the art to overcoat the novolak resin with polymethyl methacrylate. Although this improves resolution, the production of the resist involves increased production steps and increased cost. Moreover, the overcoating introduces an additional possibility of error into the photoresist composition.
Accordingly, there exists a need in the art for a simple positive-working photoresist composition which is capable of submicron resolution and can be produced in a minimum number of steps.